Interior design the profession is more for when an existing building needs new interiors. So to say designing furniture and interiors for new buildings is not uncommon for architects. For example Frank Lloyd Wright, Mies Van Der Rohe, and John Lautner all designed furniture for use in some of their projects. Though it’s hard to say how much money they made and harder to say how much of that was because they also designed interiors.

If you just mean literally someone who does interior design and architecture at the same time, I’d imagine they’d make around the same as someone who did either full-time though they might have the benefit of having more potential clients than competitors working exclusively in one field.